What U-value does a rooflight need for building regulations?

Part L of the Building Regulations governs the thermal performance of anything glazed you put into a roof. It works by setting a limiting U-value, a maximum, and the maximum it allows changes with the type of unit and with whether the house is new or already standing.

The current Approved Document L splits it out like this. In an existing dwelling, a replacement or new rooflight has to reach 2.2 W/m²K or better. A roof window, which is glazing that sits flush in the slope of a pitched roof, is grouped with windows and has to reach 1.4 W/m²K. In a brand new dwelling the standards are pulled tighter across the board, with rooflights held to 2.2 and windows and roof windows to 1.6. The rooflight figure is more relaxed than the window one for a reason: glazing that faces the sky loses heat faster than glazing set at an angle, so the regulation gives it a little more room.

The U-value is a rate of heat loss. It counts the watts that escape through every square metre of the unit for each degree of temperature difference between the room and the outside air. Lower is warmer. A figure of 2.2 lets heat out roughly twice as fast as a figure of 1.1, so the gap between just scraping the limit and specifying a good modern unit is the difference between a cold room under the glass and a comfortable one.

The single thing that trips people up here is the plane the U-value is quoted in. A rooflight lies close to horizontal, but the standard assessment for Part L measures its U-value as if it were mounted vertically, in the same plane a window would sit. A unit lying flat loses more heat than the same unit stood upright, because warm air rises straight at it, so the horizontal U-value of a rooflight is always the worse of the two numbers.

This matters when you read a specification sheet. A manufacturer may print a vertical figure, a horizontal figure, or both, and the two can be half a point apart for the same product. If a sheet quotes 1.9 vertical and 2.4 horizontal, the number that answers the Building Regulations question is the vertical one, because that is the basis the regulation is written against. Ask which plane a quoted figure is measured in before you decide whether a unit passes. A supplier confident in the product will tell you without hesitating.

Check the basis of the reading as well as the plane. A centre-pane U-value describes the glass alone and reads lower. A whole-unit U-value averages the glass with the frame and the seals, and that is the honest figure for the thing going into your roof, because a warm pane in a cold frame is only as good as its weakest part.

Meeting 2.2 is passing the exam with the lowest mark that still counts. It is worth understanding what sits below it, because the cost difference is often small and the comfort difference is not.

Modern double-glazed rooflights with a warm-edge spacer and a low-emissivity coating land somewhere between about 1.0 and 1.4 W/m²K. Step up to a triple-glazed unit and you can get down towards 0.8. The improvement is not only about the heating bill. A warmer inner pane stays above the dew point on a cold night, which is what keeps condensation from forming and running down the glass onto the reveal below. Rooflights over kitchens and bathrooms, where the air carries a lot of moisture, are the ones where a better U-value earns its keep in dry mornings rather than in pounds saved.

So treat the regulation as the line you must not cross and then decide how far past it you want to go. On a bedroom or a landing rooflight that rarely sees hard use, meeting the standard is a fair call. On a lantern over a room the family lives in, going further is usually money well spent, and our skylight installation costs page shows where the different glass build-ups land on price.

The U-value is a winter number. It tells you how fast heat leaks out on a cold night and nothing about what happens on a hot afternoon. That second question is the one Part L does not answer, because when you replace a rooflight in an existing roof the regulation caps the U-value and says nothing at all about the G-value, the amount of the sun’s heat the glass lets in.

Around here that gap decides whether a room is usable. Southend-on-Sea faces south across the Thames Estuary with an open horizon and light bouncing back off the water, so a south-facing rooflight collects sun from mid-morning until it swings west. A unit can pass Part L on its U-value, hold the winter warmth in nicely, and still turn the room below it into a greenhouse in July, because the very same sealed warmth works against you once the sun is on the glass. On a flat roof it is worse again, since horizontal glazing meets the high summer sun almost square on and orientation stops helping you.

The fix is to specify both numbers together: a U-value that satisfies the regulation for the cold months and a G-value chosen for the orientation for the warm ones. That pairing, winter loss against summer gain, is the heart of our energy-efficient skylight installation work, and it is why a south-facing rooflight over a kitchen wants a different glass specification from a north-facing one on the same house.

A new or enlarged rooflight is notifiable work. Part L covers the U-value described above, and Part K covers the safety glazing and any guarding needed where the glass is within reach or below a certain height. Both have to be satisfied and both have to be recorded with Building Control, either through a competent-person scheme or by a notification to the local authority. On our installations we make that Building Control notification to Southend-on-Sea City Council or the relevant local authority on your behalf, so the paperwork trail exists when you come to sell.

If you are specifying a unit yourself, do three things. Confirm the whole-unit U-value in the vertical plane against the 2.2 limit for a rooflight or 1.4 for a roof window in an existing home. Decide how far below the limit you want to go for the sake of a warmer, drier inner pane. Then, if the roof faces anywhere near south, ask for the G-value in the same breath so the summer is handled as deliberately as the winter. A pitched-roof unit is covered on our VELUX window installation page and a flat-roof one on flat roof skylight installation.

If you would rather hand the whole calculation over, tell us the roof orientation, whether it is flat or pitched, and the rough opening size, and we will come back with a glass specification that clears Part L and suits the light on that particular roof. Ring 01702 898232 or request a quote.